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| Book/Report | FZJ-2018-04343 |
;
1993
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/19407
Report No.: Juel-2799
Abstract: The periodic collapse of the plasma pressure inside of the inversion radius r$_{inv}$ results from the absence of a permanent adequate cross -field transport mechanism in this region. For example, trapped electron mode induced transport, which is believed to play a major role in the tokamak confining region r $\geq$ r$_{*}$ ~ r$_{inv}$, decreases rapidly at smaller radii. In the present work, a mechanism is proposed to explain how the sawtooth precursor - a transient macroscopic tearing mode - can trigger the sawtooth collapse. By deforming the original magnetic flux surfaces with q < 1 (q is the safety factor) into a cold island region and a hot spot, the precursor leads to $\textit{local}$ enhancement of the temperature and density gradients $\textit{across and near the X point}$. This, in turn, $\textit{locally}$ increases the growth rate and the fluctuation level of the trapped electron mode, as well as the associated outward transport. The azimuthallyand radially localized enhanced exhaust of energy and particles yields a depression along the field lines which, leaving the exhaust (i. e. the X point) region stretch on the hot spot's periphery. The gradient across the magnetic surfaces of the resulting outflow velocity destabilizes Kelvin-Helmholtz modes. It is shown that if $\eta_{e} $ < 2, $\eta_{e}$ = ($\partial$ ln T$_{e}$/$\partial$r)/($\partial$ ln n/$\partial$r), the latter trigger fast outward transport $\textit{across all magnetic surfaces imbedded in the hot spot}$. Collapse occurs in this way on the time-scale 2 L/c$_{s}$ where c$_{s}$ = (2 T/m$_{i}$)$^{1/2}$ is the sound speed and L is half the period of a helical field line. Outward radial transport is forbidden if $\eta_{e}$ > 2; this condition suggests a novel interpretation of the occurence of "monster" sawteeth.
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